1. Field of the Invention
The subject matter of this invention is related generally to electromagnetic contactors and more specifically to apparatus for maintaining the contacts of the contactor closed.
2. Description of the Prior Art
Electromagnetic contactors are well known in the art. A typical example may be found in U.S. Pat. No. 3,339,161 issued Aug. 29, 1967 to J. P. Connor et al., entitled "Electromagnetic Contactor" and assigned to the assignee of the present invention. Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching and similar applications. A motor-starting contactor with an overload relay system is called a motor controller. A contactor usually has a magnetic circuit which includes a fixed magnet and a movable magnet or armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with the source of voltage which may be interconnected with the main contacts of the contactor for electromagnetically accelerating the armature towards the fixed magnet thus reducing the air gap and closing the contacts. As the contactor closes, it works against the resistance of a kickout spring which operates to cause the contactor to open once again at an appropriate time. In order to maintain the contacts in the closed state in the prior art, reduced voltage is usually placed upon the electromagnet thus maintaining a small amount of electromagnetism which keeps the armature abutted against the permanent magnet and thus keeps the contacts closed. A disadvantage associated with this lies in the fact that such an arrangement is not always energy efficient. For example, over time the current flow through the windings may heat the windings of the electromagnet thus increasing the resistance thereof thus reducing the current therethrough. When this happens, the force on the magnet is reduced. Alternatively the voltage which supplies the holding current may vary within limits thus changing the current through the holding coil or winding. It would be advantageous if an efficient system could be found for maintaining the current through the holding coil at a relatively fixed value thus guaranteeing sufficient magnetomotive force in the magnetic circuit to keep the contacts closed during normal operating conditions and to furthermore provide an energy efficient way of doing that.